"I don't think we should be rolling out the red carpet to the President of the USA in the circumstances where his policies go against everything we stand for," he said. On Tuesday morning, Khan took on a more dismissive tone, telling an ITV interviewer of Trump, "Look, it takes two to tango, and I am not tangoing with this guy. I've got better things to do."

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It might be too late.

Trump is not one to forget a slight, even when his own actions or freewheeling criticism invites, or demands, some kind of response. During the 2016 US campaign, Khan -- the first Muslim mayor of London or any other major Western capital -- denounced Trump's proposed Muslim ban. At that, the rivalry was established. In March, a different Trump took aim at Khan. Donald Trump Jr., in the aftermath of the attack on Westminster Bridge, took a shot at Khan's past remarks about the terror threat to big cities.

Trump and his team established this pattern of behavior early on in the Republican primary campaign: Zero in on some perceived offense or would-be ideological opponent, then start swinging. As an electoral tactic, it was a winner. Trump bullied the GOP field on social media and blitzed Hillary Clinton in the fall. Since becoming president, he has fired one appointee and rejected his secretary of state's choice for a deputy because of their respective pre-election criticisms.

Trump's latest onslaught began early Sunday, when he singled out five words from an interview Khan had given earlier that morning informing local viewers what to expect in the coming days.

"Londoners will see an increased police presence today and over the course of the next few days -- no reason to be alarmed," Khan said. "One of the things the police and all of us need to do is make sure we're as safe as we possibly can be." (Hear it for yourself at the 3:05 mark.)

Again, even a cursory reading of the mayor's remarks reveal how obviously Trump had misrepresented them. Khan had been clear. There will be more police on the streets. Don't stress over that. It is a precaution. Still, Trump lashed out. It was only the beginning.

A little more than two hours later, the White House social media director Dan Scavino piled on, taunting Khan.

"@MayorofLondon - refer to below tweet 13 months ago, after you criticized ... now President @realDonaldTrump --- and WAKE UP!!!!"

Not a day had passed since the attacks and a top Trump communications official -- an assistant to the President -- was using his official White House account to berate the London mayor over comments from, as he noted, more than a year earlier. The institutional memory in the Trump White House seems predetermined to prioritize snubs and affronts.

By Monday morning, Kellyanne Conway, a senior adviser to the President, was taking her shots. Asked on NBC's "Today" show if Trump owed Khan an apology for taking his statement so clearly out of context, Conway dodged the question, then chided an interviewer over "this obsession with covering everything (Trump) says on Twitter."

Afterward, Conway posted a clip of the exchange -- on Twitter -- with the following comment: "Terrorists again slaughter innocents, TelegraphUK reports neighbors warned authorities about killer, but @POTUS should apologise?"

As the hubbub grew, Trump, as has become common practice, doubled down. At a little before 10 a.m., after what had apparently been a morning of watching and reading press coverage of the spat, he went after Khan. Again.

"Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his 'no reason to be alarmed' statement," he tweeted "(Mainstream media) is working hard to sell it!"

Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his "no reason to be alarmed" statement. MSM is working hard to sell it!

The President had spoken. And in the aftermath of this, the third terror attack on British soil in less than three months, he had chosen to use his platform to troll the mayor of London -- and potentially settle an old score.